Electromagenetic compressor



Feb. 3, 1959 A. RYBA "2,872,101

ELECTROMAGNETIC COMPRESSOR Filed Dec. 19, 1956 3 Sheets-Sheet 1 FIG. a y I) A. RYBA ELECTROMAGNETIC COMPRESSOR Feb. 3, 1959 7 Filed Dec. 19, 1956 3 Sheets-Sheet 2 Fla. 5

mu Q m Q will mm|- INVENTOR.

Feb. 3, 1959 A I ELECTROMAGNETIC COMPRESSOR s Sheets-Sheet fa Filed Dec. 19, 1956 g aaa aamv FIG.

1 0 m m w 2,872,101 ELECTROMAGNETIC COMTRESSOR Anton Ryba, Bolzano, Italy, assignor to Stempel-Hermetik G. in. b. H., Frankfurt am Main, Germany Application December 19, 1956, Serial No. 629,335 Claims priority, application Austria December 19, 1955 15 Claims. (Cl. 230-55) The present invention relates to compressors.

More particularly, the present invention relates to electromagnetic compressors wherein a piston of the compressor oscillates in accordance with the frequency of the current applied to the electromagnet of the compressor. Conventional compressors of this type have several disadvantages such as poor efliciency, difiiculty in accurately limiting the stroke of the piston, and the necessity of using relatively large current.

One of the objects of the present invention is to overcome the above drawbacks by providing an electromagnetic compressor capable of accurately limiting the stroke of the piston thereof.

Another object of the present invention is to provide a compressor of the above type which operates with far less noise than a conventional compressor of this type.

An additional object of the present invention is to provide a structure capable of accomplishing the above objects and composed of simple, ruggedly constructed elements which are very reliable in operation.

With the above objects in view the present invention mainly consists of an electromagnetic compressor which includes a housing carrying an electromagnetic means. A cylinder is carried by the housing and a piston member is slidable in the cylinder. An armature member is fixed to the piston member, so that these members move together, and the armature member cooperates with the electromagnetic means to be reciprocated thereby. A conduit and valve means cooperates with the cylinder and piston member for directing a fluid into the cylinder ahead of the piston member during suction strokes of the latter and for directing compressed fluid out of the cylinder to a desired location during pressure strokes of the piston member. A spring means is carried by the housing and cooperates with either the armature member of the piston member for preventing free oscillation of these members, and this spring means includes portions which engage each other to limit the stroke of the piston member.

The novel features which are considered as characteristic for the invention are set forth in particular in the a appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings, in which:

Fig. 1 is a sectional elevational view of a compressor according to the present invention, the section of Fig. 1 being taken along line AOB of Fig. 3;

Fig. 2 is a partly sectional plan view of the compressor of Fig. 1 as seen from the underside thereof with the cover removed;

Fig. 3 is a sectional view of the compressor of Fig. 1 taken along line C-C of Fig. l with the piston eliminated from Fig. 3;

Fig. 4 is an axial, sectional elevational view of a central portion of a compressor similar to that of Fig. 1

and including a spring which is both tensioned and compressed;

Fig. 5 is a fragmentary axial sectional elevational view of the central portion of a double acting compressor according to the present invention;

Figs. 6-10 are sectional elevational views of dilferent types of coil springs which may be used in the compressor of the invention;

Fig. 11 is a sectional elevational view of a spring of the present invention which is in the form of a column of pairs of opposed dish springs;

Fig. 12 is a fragmentary sectional elevational View showing an arrangement where coil and dish springs are combined; and

Fig. 13 is a sectional elevational view of an arrangement which includes a combination of springy discs and a coil spring.

The compressor illustrated in Figs. l3 includes a housing 1 of non-magnetic material having a cover 2 which is fixed to the housing 1 by means of screws 3. Groups of U-shaped laminations 4 are carried by the housing 1 in grooves formed in the bottom and outer side walls of the housing 1 in the interior thereof, and these grooves of laminations extend radially with respect to the axis of the housing 1 and are uniformly distributed about this axis, the groups of laminations forming an annular space in which the exciting coil 5 is received.

The groups of laminations 4 together with the annular coil 5 are covered by a flat plate 6 in which are embedded, during the casting of plate 6, the elongated return pole pieces in the form of groups of laminations 7 which respectively extend across and over the groups of laminations 4. The upper ends of the inner legs of the groups of laminations 4 and the inner ends of the groups of laminations 7 are spaced from each other in the manner illustrated in Fig. 1 to provide a gap in which an armature is freely movable in a vertical direction, as viewed in Fig. 1.

The armature 9 is rigidly fixed to a piston 8 which slides within a cylinder 10 which is pressed into the housing ll. The cylinder lliis provided with a separate, movable bottom wall 111 which simultaneously serves as a pressure valve and which is held against the bottom face of the cylinder it by a small leaf spring 12 carried by a bottom cover plate 14 which covers the bore of the housing 1 which receives the cylinder 10 and which is fixed to the housing 1 by the screws 13.

The interconnected piston 3 and armature 9 are connected to the housing 1 by a spring means which, in the example shown in Fig. 1, includes a pair of coil springs 15 and 36 located in the interior of the piston 8 and respectively abutting against opposite faces of a flange 18 which is fixed to a rod 17. The spring 15 can have any of the constructions shown in Figs. 6-10, while the spring to can be of conventional construction. The rod 17 is fixed at its upper end, as viewed in Fig. l, to the'center of a flexible, preferably springy disc or plate 20 which is gripped at its periphery between the cover 2 and an annular flange located at the periphery of the plate 6 and extending upwardly therefrom, as viewed in'Fig. 1.

A suitable conduitand valve means is provided for leading a fluid into the cylinder 10 ahead of the piston 3 during suction strokes of the latter and for leading compressed fluid out of the cylinder lit to a desired location during compression strokes of the piston 8. Thisconduit and valve means includes a suction conduit 21 shown at the lower ieft portion of Fig. l and extending from the ex terior of the housing 1 into the interior thereof and then axially along the interior of the housing Ti along a path 10- 0 cated beyond the coil 5 upwardly to the space beneath the disc 26?. This space beneath the disc 20 communicates with the interior of the piston 8 through an annua b lar gap formed between the inner periphery of the armature 9 and the rod 17. The head end of the piston 8 is formed with a plurality of openings 22 arranged in a circle,. and a springy plate 23 is fixed to the bottom face of the piston 8 and has springy fingers respectively covering the openings 22, so that these fingers act as suction valves, and during upward movement of the piston 8, as viewed in Fig. l, thefluid will flow through the suction valves into the cylinder 10 to the space ahead of the piston 8, this latter space being shown at 24. The pressure valve plate 11 is located at the bottom of the chamber 24, and the chamber 25 beneath the plate 11 communicates through the annular space between the housing 1 and the bottom reduced end of the cylinder 1 clearly shown in Fig. 1, with the radially extending pressure conduit 27 which extends through and beyond the housing 1 to deliver the compressed fluid to a desired location.

The electrical leads 28 for the coil 5 extend through radial bores of the housing 1 formed in portions thereof located between groups of laminations 4. In order to carry away the relatively small amount of heat lost during operation of the compressor, the housing 1 is provided with the cooling fins 30 extending from the bottom and side walls of the housing 1, and these fins are adequate for this purpose.

The above described compressor operates as follows:

The coil 5 is connected in series with a rectifier to a. single phase source of current, so that only half waves flow through the coil to excite the latter periodically with the result that the armature-piston assembly is set into oscillation, and the magnetic force produces the compression stroke of the piston. During this pressure stroke the armature 9 is pulled downwardly, as viewed in Fig. l, and the piston 8 moves dowwardly with the suction valves 23 closed. When the fluid in the chamber 24 has reached a predetermined degree of compression, the plate 11 automatically moves downwardly from its seat against the force of the spring 12, and the compressed fluid flows out of the chamber 24 into the chamber 25 and through the pressure conduit 27.

Before the end of the pressure stroke, that is, before the piston 8 reaches the bottom end of the cylinder 10, because of the, preselected spring constant of the spring 15 and also of the plate 26 if desired, the force with which these springs retard the movement of the piston and armature increases at an accelerated rate and the convolutions of the spring 15 finally come into engagement with each other. In this way the stroke of the piston is accurately limited without shock and without noise, and the spring means is capable of absorbing the largest amount of energy required for a compression stroke of the piston while actually retarding the piston movement only during a small fraction of its useful stroke.

If the compressor is used in a refrigerating machine where the fluid to be compressed is a mixture of gas and oil, then the spring 15 which is located in the path of the fluid is covered with a coating of oil which is carried along with the gas in the form of a vapor, and the result is that the mutual engagement of the spring convolutions at the end of the piston stroke is even further damped.

The succeeding suction stroke then results from the spring force which now acts alone, and this spring force must be capable of providing not only the required acceleration of the masses but also of overcoming the suction and friction resistance, since the current half waves occur almost simultaneously with and for the duration of the compression strokes.

The counter spring 16 is provided to effect a precise change in the direction of movement of the piston and armature at the end of the suction stroke, and this spring 16 stores a part of the kinetic energy of the pressure stroke and gives up this energy to the oscillating masses during the suction stroke. The springs 15 and 16 are designed so as to provide a resultant force which gives a harmonic oscillation of the masses. The oscillating system corresponds advantageously to the desired frequency.

The interior of the housing 1 is provided with mannular oil chamber 31 surrounding the inner tubular wall of the housing which receives the cylinder 10, and the oil from chamber Slflows through a bore 32 to the outer face of the piston 8 in order to lubricate the latter. The chamber 31 is supplied by oil settling from the oil vapor which is carried along with the gas.

Fig. 4 shows a different embodiment of the spring means of the invention, and except for the difference in the spring means the structure of Fig. 4 is identical with that of Fig. 1. According to the embodiment shown in Fig. 4 a single spring 40 performs the functions of the two springs 15 and 16 and must therefore be stressed both in tension and compression. The spring 49 is therefore provided at its upper end with outer threads threadedly engaging threads formed in the interior of the piston 38, so that in this way the spring 40 is connected with the piston 38, and at its lower end the spring 40 has inner threads threadedly engaging threads at the bottom end of the rod 41, so that in this way the spring 40 is also connected to the rod 41. Piston 38 and rod 41 respectively correspond to piston 8 and rod 17 of Fig. l. The rod 41 is fixed to the center of the plate 20 in the same way as the rod 17.

Fig. 5 illustrates only the central portion of a double acting compressor. This compressor is provided with a disc 52 corresponding to the disc 20, and electromagnetic structures identical with that of Figs. 1-3 are symmetrically arranged on opposite sides of the disc 52. This disc 52 is located between a pair of springs 50 and 50' which cooperate with the disc 52 to form the spring means of the embodiment of Fig. 5, and these springs 50 and 50 respectively press against the pair of opposed I pistons 48 which are symmetrically arranged in a coaxial manner on opposite sides of the disc 52 and which are fixedly connected to each other by the rod 51 which extends with clearance through the coaxial springs 50 and 50 and through an opening in the center of the disc 52. Each of the springs 50 and 50' has a construction similar to spring 15 of Fig. 1. The suction valves 54 and the pressure valves 55 are also completely analogous to and identical with the suction and compression valves of the embodiment of Fig. l. The housing of the embodiment of Fig. 5 includes a pair of symmetrical housing portions interconnected with each other by an unillustrated centering ring and bolts and nuts. Both of thesuction and pressure connections are connected with common conduits.

The operation of the compressor of Fig. 5 is essentially the same as that of Fig. 1 except that the pressure stroke of one of the pistons 48 overlaps the suction stroke of the other of these pistons, and thus the springs 50 and 50' are completely identical since a pressure stroke occurs in both directions of movement of .the pistons,

suitable armatures identical with the armature 9 being fixed respectively to the pistons 48 for movement therewith. With the arrangement of Fig. 5 the coils of the electromagnet means are respectively connected in series with rectifiers which respectively permit opposed half waves to pass through the respective coils.

Instead of conventional springs, springs as illustrated in Figs. 6-11 may advantageously be used in any of the above described compressors to replace the coil springs thereof. Fig. 7 shows a coil spring whose successive convolutions are located at gradually increasing distances from each other from the top to the bottom end of the spring of Fig. 7. Such a spring can have any desired spring constant which will continually increase during the compression of the spring. It will be noted that with such a spring, the engagement of the convolutions moves continuously from the end of the spring where the convolutions are nearest to each other toward the end of the spring where the convolutions are furthest from each other, during-compression of the spring, and this operation results in an absolutely shock and noise free absorption of the forces.

Figs. 6 and 8 show how the same result can be obtained with a different construction, that is, by providing a spring made from a bar of continuously changing cross section. The difference of the embodiments of Figs. 6 and 8 with respect to Fig. 7 resides only in the saving of material, since the convolutions at the end of the spring of smallest cross section need not have the stiffness of the convolutions at the end of the spring of largest cross section. It is also possible to combine both features by providing a spring of constantly changing cross section and constantly changing distance between the successive convolutions.

Fig. 9 shows another embodiment of a spring having the same properties as the springs of Figs. 6-8, and the desired result is obtained with the embodiment of Fig. 9 by continuously changing the diameters of the successive convolutions. With respect to Figs. 6 and 8, it will be noted that in Fig. 6 the cross section of the convolutions changes by providing the outer surface of the spring within a frusto-conical configuration, while in Fig. 8 the axial cross section of the convolutions gradually changes.

Fig. 10 shows a spring made up of' a series of coaxial independent springs which respectively have different cross sections and difierent distances between their convolutions. The spring constant of this spring is a broken line which however is suitable for usein the compressor of the invention. The individual springs of the column are separated from each other and centered with respect to each other by suitable separating discs in the form of the pair of rings shown between the three springs of Fig. 10.

Fig. 11 shows a spring in the form of a column of pairs of. opposed annular dish springs. The several pairs of dish springs have ditferent characteristics and the degree of difference from one pair of dish springs to the next is relatively small toprovide a smooth operation. The elements with the steepest spring constants have spring constant curves which curve upwardly.

It is pointed out that the term spring constant refers to the force required to change the length of a spring through a given unit of length. Of course, with the springs of the invention this force varies with the extent to which the spring is stressed.

Fig. 12 shows a compressor similar to that of Figs. 1-3 wherein the spring 15 is replaced by a coil spring 6511 whose successive convolutions are spaced at increasing distances from each other in combination with a pair of annular dish springs 61 having a spring constant greater than that of the spring 60. The pair of dish springs 61 are capable of performing the spring function of the plate 26, so that with the embodiment of Fig. 12 the plate which corresponds to the plate of Fig. 1 need not be resilient.

With the arrangement shown in Fig. 13, the spring 15 of Fig. 1 is replaced by, a conventional coil spring 62 connected in parallel to a spring composed of a column of annular, thin, springy plates 63 of the same or difrer ent spring constants. With this embodiment the convolutions of the spring 62 do not engage each other, while the plates 63 cometo lie completely against each other simultaneously if they have identical spring constants and in succession if they have successively increasing spring constants, so that with this arrangement the inner spring 63 limits the stroke while the outer stpring 62 stores the energy necessary for the suction stroke.

Thus, the spring 63 acts mainly to limit the stroke while the frequency of'the currentapplied to'the electromagnet means, and more steps or continuous changes in the spring constant will berequired as the frequency is greater, since with increasing frequency the possibility of shock resulting from simultaneous engagement of the several spring portions with each other increases. An other factor in the choice of the number of diiferent elements of the spring means is the dilference between a lubricated compressor or a non-lubricated compressor. Where the compressor is lubricated, the oil film on the spring means can delay the expansion of the spring means significantly in accordance with the viscosity of theoil, so that particularly at high frequencies the suction stroke may not yet be completed when the electromagnet means is again excited for the succeeding pressure stroke. In such a case,the smallest number of spring elements will be chosen, and the last of these elements will preferably be given a very steep spring constant. Where coil springs are used, they advantageously have a rectangular cross section in order to provide between the convolutions an area of contact which is substantially the same as that which can. be provided with dish springs.

It is possible to use, in addition to or instead of coil springs or dish springs or combinations thereof, a series of thin springy discs. The disc 20 advantageously has a high spring constant; Itis alsopossible, however, to provide a plate corresponding to the spring 26 and having practically no resiliency and'to provide a high spring constant at another element of the spring means, as was described above in connection with Fig. 12.

The stroke limiting features of the above described spring means of the invention may be used-in any compressor or the like, and the compressor can take any form and may, for example, be rectangular rather than ring-shaped.

It will be understood that each of the elements described above, or two or. more together, may also find a useful application in other types of compressors differing from the types described above.

While the invention has been illustratedand described as embodied in electromagnetic compressors, it is not intended to be limited to the details shown, since various modifications and. structural changes may be made without departing in any way from the spirit of the present invention.

Without further-analysis, the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge; readily adapt it for various'applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of'this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalenceof the following claims.

What is'claimed as new and desired to be secured by Letters Patent is:v

1. An electromagnetic compressor comprising, in combination, a housing; electromagnetic means carried by said housing; a cylinder. carried by said housing; a piston member .slidable in said cylinder; an armature member fixed to said piston member, so that said members move together, and said armature member cooperating with said electromagnetic means to be reciprocated by the latter; conduit and valvemeans cooperating with said cylinder and piston member for directing a fluid into said cylinder ahead of said piston member during suction member slidable in said cylinder; an armature member fixed to said piston member, so that said members move togetherjand said armature member cooperating with said electromagnetic means to be reciprocated by the latter; conduit and valve means cooperating with said cylinder and piston member for directing a fluid into said cylinder ahead of said piston member during suction strokes of the latter and for directing compressed'fluid out of said cylinder to a desired location during pressure strokes of said piston member; and spring'means carried by said housing and cooperating with one of said members for preventing free oscillation of said members, said spring means including a spring having portions which engage each other to limit the pressure stroke of said piston member.

3. An electromagnetic compressor comprising, in com bination, a housing; electromagnetic means carried by said housing; a cylinder carried by said housing; a piston member slidable in said cylinder; an armature member fixed to said piston member, so that said members move together, and said armature member cooperating with said electromagnetic means to be reciprocated by the latter; conduit and valve means cooperating with said cylinder and piston member for directing a fluid into said cylinder ahead of said piston member during suction strokes of the latter and for directing compressed fluid out of said cylinder to a desired location during pressure strokes of said piston member; and spring means carried.

by said housing and cooperating with one of said members for preventing free oscillation of said members, said spring means including a plurality of springs at least one of which has a spring constant greater than that of the others.

4. An electromagnetic compressor comprising, in combination, a housing; electromagnetic means carried by said housing; a cylinder carried by said housing; a piston member slidable in said cylinder; an armature member fixed to said piston member, so that said members move together, and said armature member cooperating with said electromagnetic means to be reciprocated by the latter; conduit and valve means cooperating with said cylinder and piston member for directing a fluid into said cylinder head of said piston member during suction strokes of the latter and for directing compressed fluid out of said cylinder to a desired location during pressure strokes of said piston member; and spring means carried by said housing and cooperating with one of said members for preventing free oscillation of said members, said spring means including portions which engage each other to limit the stroke of said piston member and said spring means including at least one spring which has a spring constant which increases during compression of said one spring.

5. An electromagnetic compressor comprising, in combination, a housing, electromagnetic means carried by said housing; a cylinder carried by said housing; a piston member slidable in said cylinder; an armature member fixed to said piston member, so that said members move together, and said armature member cooperating With said electromagnetic means to be reciprocated by the latter; conduit and valve means cooperating with said cylinder and piston member for directing a fluid into said cylinder ahead of said piston member during suction strokes of the latter and for directing compressed fluid out of said cylinder to a desired location dur- '6. An electromagnetic compressor comprising, in combination, a housing, electromagnetic means carried by said housing; a cylinder carried by said housing; a piston member slidable in said cylinder; an armature member fixed to said piston member, so that said members move together, and said armature member cooperating with said electromagnetic means to be reciprocated by the latter; conduit and valve means cooperating with said cylinder and piston member for directing a fluid into said cylinder ahead of said piston member during suction strokes of the latter and for directing compressed fluid out of said cylinder to a desired location during pressure strokes of said piston member; and spring means carried by said housing and cooperating with one of said members for preventing free oscillation of said members, said spring means including poring pressure strokes of said piston member; and spring means carried by said housing and cooperating with one of said members for preventing free oscillation of said members, said spring means including portions which engage each other to limit the stroke of said piston member and said spring means including at least one coil spring of constant cross section whose convolutions are spaced from each other, when said one spring is unstressed, by distances which gradually increase from one end of said one spring to the other end thereof.

tions which engage each other to limit the stroke of said piston member and said spring means including at least one coil spring having convolutions of gradually increasing cross section.

7. An electromagnetic compressor comprising, in combination, a housing; electromagnetic means carried by said housing; a cylinder carried by said housing; a piston member slidable in said cylinder; an armature member fixed to said piston member, so that said members move together, and said armature member cooperating with said electromagnetic means to be reciprocated by the latter; conduit and valve means cooperating with said cylinder and piston member for directing a fluid into said cylinder ahead of said piston member during suction strokes of the latter and for directing compressed fluid out of said cylinder to a desired location during pressure strokes of said piston member; and spring means carried by said housing and cooperating with one of said members for preventing free oscillation of said members, said spring means including portions which engage each other to limit the stroke of said piston member and said spring means including a spring assembly made up of a series of independent coaxial coil springs maintained separate from each other and in coaxial relationship with respect to each other by a plurality of separating and centering elements respectively located between said series of springs.

8. An electromagnetic compressor comprising, in combination, a housing; electromagnetic means carried by said housing; a cylinder carried by said housing; a piston member slidable in said cylinder; an armature member fixed to said piston member, so that said members move together, and said armature member cooperating with said electromagnetic means to be reciprocated by the latter; conduit and valve means cooperating with said cylinder and piston member for directing a fluid into said cylinder ahead of said piston member during suction strokes of the latter and for directing compressed fluid out of said cylinder to a desired location during pressure strokes of said piston member; and spring means carried by said housing and cooperating with one of said members for preventing free oscillation of said members, said spring means including portions which engage each other to limit the stroke of said piston member and said spring means including at least one spring composed of acolumn of dish springs of which at least one has a spring constant greater than'the others.

9. An electromagnetic compressor comprising, in combination, a housing; electromagnetic means carried by said housing; a cylinder carried by said housing; a piston member slidable in said cylinder; an armature member fixed to said piston member, so that said members move together, and said armature member cooperating with said electromagnetic means to be reciprocated by the latter; conduit and valve meanscooperating with said cylinder and piston member for directing a fluid into said cylinder ahead of said piston member during suction strokes of the latter and for directing compressed fluid out of said cylinder to a desired location during pressure strokes of said piston member; and spring means carried by said housing and cooperating with one of said members for preventing free oscillation of said members, said spring means including portions which engage each other to limit the stroke of said piston member and said spring means including a plurality of individual springs.

10. An electromagnetic compressor as recited in claim 1 and wherein said spring means prevents free oscillation of said piston member and wherein said spring means includes one spring acting mainly to yieldably resist free oscillation of said piston member and another spring connected in parallel with said one spring and acting mainly to limit the stroke of said piston member.

11. An electromagnetic compressor as recited in claim 1 and wherein said spring means prevents free oscillation of said piston member and wherein said spring means includes a coil spring which is both tensioned and compressed during reciprocation of said piston member.

12. An electromagnetic compressor as recited in claim 1 and wherein said spring means prevents free oscillation of said piston member and wherein said spring means includes a springy plate connected directly to said housing and at least one additional spring cooperating with said plate and said one member for yieldably resisting movement of the latter.

13. An electromagnetic compressor as recited in claim 1 and wherein said spring means prevents free oscillation of said piston member and wherein said spring means in cludes at least one coil spring having convolutions of gradually increasing diameters, respectively.

14. A double electromagnetic compressor comprising, in combination, a housing; a pair of opposed electromagnetic means symmetrically carried by said housing; a pair of opposed cylinders also symmetrically carried by said housing; a pair of piston members respectively slidable in said cylinders; a pair of armature members respectively fixed to said piston members and said armature members respectively cooperating with said pair of electromagnetic means to be reciprocated by the latter; connecting means interconnecting one of said pair of members for movement together; conduit and valve means cooperating with said cylinders and piston members for directing a fluid into said cylinders ahead of said piston members during suction strokes of the latter and for directing compressed fluid out of said cylinders to a desired location during pressure strokes of said piston members; a plate carried by said housing; and a pair of spring means cooperating with said plate and connecting means and respectively with said one pair of members for preventing free oscillation of said pair of piston members, said pair of spring means each having portions which engage each other to limit the stroke of said piston members.

15. An electromagnetic compressor as recited in claim 1 and wherein said spring means consists of at least one spring whose convolutions are all of the same size.

References Cited in the file of this patent UNITED STATES PATENTS 2,054,097 Replogle Sept. 15, 1936 2,258,586 Glassing Oct. 14, 1941 2,630,760 Ryba Mar. 10, 1953 2,659,310 Ryba Nov. 17, 1953 FOREIGN PATENTS 262,884 Switzerland Oct. 17, 1949 

